dart2js cps: More type rules in type propagation.

tests/compiler/dart2js/cps_ir/expected/operators_4.js

 // Expectation for test: 
 // var x = 1;
 // get foo => ++x > 10;
 // main() { print(foo && foo); }
 
 function() {
   var v0 = $.x + 1, line;
   $.x = v0;
   L0: {
     if (v0 > 10) {
       $.x = v0 = $.x + 1;
       if (v0 > 10) {
         line = "true";
         break L0;
       }
-      v0 = false;
-    } else
-      v0 = false;
-    line = v0 ? String(v0) : "false";
+    }
+    line = "false";
   }
   if (typeof dartPrint == "function")
     dartPrint(line);
   else if (typeof console == "object" && typeof console.log != "undefined")
     console.log(line);
   else if (!(typeof window == "object")) {
     if (!(typeof print == "function"))
       throw "Unable to print message: " + String(line);
     print(line);
   }
 }